Add a governor extension that implements a proposal guardian

contracts/governance/extensions/GovernorProposalGuardian.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {Governor} from "../Governor.sol";
+
+/**
+ * @dev Extension of {Governor} which adds a proposal guardian that can cancel proposals at any stage of their lifecycle.
+ *
+ * Note: if the proposal guardian is not configured, then proposers take this role for their proposals.
+ */
+abstract contract GovernorProposalGuardian is Governor {
+    address private _proposalGuardian;
+
+    event ProposalGuardianSet(address oldProposalGuardian, address newProposalGuardian);
+
+    /**
+     * @dev Override {IGovernor-cancel} that implements the extended cancellation logic.
+     * * proposal guardian can cancel any proposal at any point in the lifecycle.
+     * * if no proposal guardian is set, proposer can cancel their proposals at any point in the lifecycle.
+     * * if the proposal guardian is set, proposer keep their ability to cancel during the pending stage (default behavior).
+     */
+    function cancel(
+        address[] memory targets,
+        uint256[] memory values,
+        bytes[] memory calldatas,
+        bytes32 descriptionHash
+    ) public virtual override returns (uint256) {
+        address caller = _msgSender();
+        address authority = proposalGuardian();
+
+        if (authority == address(0)) {
+            // if there is no proposal guardian
+            // ... only the proposer can cancel
+            // ... no restriction on when the proposer can cancel
+            uint256 proposalId = getProposalId(targets, values, calldatas, descriptionHash);
+            address proposer = proposalProposer(proposalId);
+            if (caller != proposer) revert GovernorOnlyProposer(caller);
+            return _cancel(targets, values, calldatas, descriptionHash);
+        } else if (authority == caller) {
+            // if there is a proposal guardian, and the caller is the proposal guardian
+            // ... just cancel
+            return _cancel(targets, values, calldatas, descriptionHash);
+        } else {
+            // if there is a proposal guardian, and the caller is not the proposal guardian
+            // ... apply default behavior
+            return super.cancel(targets, values, calldatas, descriptionHash);
+        }
+    }
+
+    /**
+     * @dev Getter that returns the address of the proposal guardian.
+     */
+    function proposalGuardian() public view virtual returns (address) {
+        return _proposalGuardian;
+    }
+
+    /**
+     * @dev Update the proposal guardian's address. This operation can only be performed through a governance proposal.
+     *
+     * Emits a {ProposalGuardianSet} event.
+     */
+    function setProposalGuardian(address newProposalGuardian) public virtual onlyGovernance {
+        _setProposalGuardian(newProposalGuardian);
+    }
+
+    /**
+     * @dev Internal setter for the proposal guardian.
+     *
+     * Emits a {ProposalGuardianSet} event.
+     */
+    function _setProposalGuardian(address newProposalGuardian) internal virtual {
+        emit ProposalGuardianSet(_proposalGuardian, newProposalGuardian);
+        _proposalGuardian = newProposalGuardian;
+    }
+}

contracts/mocks/governance/GovernorProposalGuardianMock.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+import {Governor} from "../../governance/Governor.sol";
+import {GovernorSettings} from "../../governance/extensions/GovernorSettings.sol";
+import {GovernorCountingSimple} from "../../governance/extensions/GovernorCountingSimple.sol";
+import {GovernorVotesQuorumFraction} from "../../governance/extensions/GovernorVotesQuorumFraction.sol";
+import {GovernorProposalGuardian} from "../../governance/extensions/GovernorProposalGuardian.sol";
+
+abstract contract GovernorProposalGuardianMock is
+    GovernorSettings,
+    GovernorVotesQuorumFraction,
+    GovernorCountingSimple,
+    GovernorProposalGuardian
+{
+    function proposalThreshold() public view override(Governor, GovernorSettings) returns (uint256) {
+        return super.proposalThreshold();
+    }
+
+    function cancel(
+        address[] memory targets,
+        uint256[] memory values,
+        bytes[] memory calldatas,
+        bytes32 descriptionHash
+    ) public override(Governor, GovernorProposalGuardian) returns (uint256) {
+        return super.cancel(targets, values, calldatas, descriptionHash);
+    }
+}

test/governance/extensions/GovernorProposalGuardian.test.js

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+const { ethers } = require('hardhat');
+const { expect } = require('chai');
+const { loadFixture } = require('@nomicfoundation/hardhat-network-helpers');
+const { impersonate } = require('../../helpers/account');
+
+const { GovernorHelper } = require('../../helpers/governance');
+const { ProposalState } = require('../../helpers/enums');
+
+const TOKENS = [
+  { Token: '$ERC20Votes', mode: 'blocknumber' },
+  { Token: '$ERC20VotesTimestampMock', mode: 'timestamp' },
+];
+
+const name = 'Proposal Guardian Governor';
+const version = '1';
+const tokenName = 'MockToken';
+const tokenSymbol = 'MTKN';
+const tokenSupply = ethers.parseEther('100');
+const votingDelay = 4n;
+const votingPeriod = 16n;
+const value = ethers.parseEther('1');
+
+describe('GovernorProposalGuardian', function () {
+  for (const { Token, mode } of TOKENS) {
+    const fixture = async () => {
+      const [owner, proposer, voter1, voter2, voter3, voter4, other] = await ethers.getSigners();
+      const receiver = await ethers.deployContract('CallReceiverMock');
+
+      const token = await ethers.deployContract(Token, [tokenName, tokenSymbol, tokenName, version]);
+      const mock = await ethers.deployContract('$GovernorProposalGuardianMock', [
+        name, // name
+        votingDelay, // initialVotingDelay
+        votingPeriod, // initialVotingPeriod
+        0n, // initialProposalThreshold
+        token, // tokenAddress
+        10n, // quorumNumeratorValue
+      ]);
+
+      await impersonate(mock.target);
+      await owner.sendTransaction({ to: mock, value });
+      await token.$_mint(owner, tokenSupply);
+
+      const helper = new GovernorHelper(mock, mode);
+      await helper.connect(owner).delegate({ token, to: voter1, value: ethers.parseEther('10') });
+      await helper.connect(owner).delegate({ token, to: voter2, value: ethers.parseEther('7') });
+      await helper.connect(owner).delegate({ token, to: voter3, value: ethers.parseEther('5') });
+      await helper.connect(owner).delegate({ token, to: voter4, value: ethers.parseEther('2') });
+
+      return { owner, proposer, voter1, voter2, voter3, voter4, other, receiver, token, mock, helper };
+    };
+
+    describe(`using ${Token}`, function () {
+      beforeEach(async function () {
+        Object.assign(this, await loadFixture(fixture));
+
+        // default proposal
+        this.proposal = this.helper.setProposal(
+          [
+            {
+              target: this.receiver.target,
+              value,
+              data: this.receiver.interface.encodeFunctionData('mockFunction'),
+            },
+          ],
+          '<proposal description>',
+        );
+      });
+
+      it('deployment check', async function () {
+        expect(await this.mock.name()).to.equal(name);
+        expect(await this.mock.token()).to.equal(this.token);
+        expect(await this.mock.votingDelay()).to.equal(votingDelay);
+        expect(await this.mock.votingPeriod()).to.equal(votingPeriod);
+      });
+
+      describe('set proposal guardian', function () {
+        it('from governance', async function () {
+          const governorSigner = await ethers.getSigner(this.mock.target);
+          await expect(this.mock.connect(governorSigner).setProposalGuardian(this.voter1.address))
+            .to.emit(this.mock, 'ProposalGuardianSet')
+            .withArgs(ethers.ZeroAddress, this.voter1.address);
+          expect(this.mock.proposalGuardian()).to.eventually.equal(this.voter1.address);
+        });
+
+        it('from non-governance', async function () {
+          await expect(this.mock.setProposalGuardian(this.voter1.address))
+            .to.be.revertedWithCustomError(this.mock, 'GovernorOnlyExecutor')
+            .withArgs(this.owner.address);
+        });
+      });
+
+      describe('cancel proposal during active state', function () {
+        beforeEach(async function () {
+          await this.mock.$_setProposalGuardian(this.voter1.address);
+          await this.helper.connect(this.proposer).propose();
+          await this.helper.waitForSnapshot(1n);
+          expect(this.mock.state(this.proposal.id)).to.eventually.equal(ProposalState.Active);
+        });
+
+        it('from proposal guardian', async function () {
+          await expect(this.helper.connect(this.voter1).cancel())
+            .to.emit(this.mock, 'ProposalCanceled')
+            .withArgs(this.proposal.id);
+        });
+
+        it('from proposer when proposal guardian is non-zero', async function () {
+          await expect(this.helper.connect(this.proposer).cancel())
+            .to.be.revertedWithCustomError(this.mock, 'GovernorUnexpectedProposalState')
+            .withArgs(
+              this.proposal.id,
+              ProposalState.Active,
+              GovernorHelper.proposalStatesToBitMap([ProposalState.Pending]),
+            );
+        });
+
+        it('from proposer when proposal guardian is zero', async function () {
+          await this.mock.$_setProposalGuardian(ethers.ZeroAddress);
+          await expect(this.helper.connect(this.proposer).cancel())
+            .to.emit(this.mock, 'ProposalCanceled')
+            .withArgs(this.proposal.id);
+        });
+      });
+    });
+  }
+});